This invention relates to lasers, and in particular to Raman lasers. Even more particularly, the invention relates to solid state Raman lasers and amplifiers pumped noncollinearly.
The Raman scattering process involves frequency shifting of light, and perhaps changing the light""s propagation direction, as light passes through a Raman material and scatters from lattice vibrations. The energy difference between the incident and scattered light creates or annihilates a vibration within a molecule (localized vibration) or lattice mode (delocalized vibration) of the material. These vibrations are quantized, and termed phonons, in analogy to photons. Phonons are similar to photons in that they are wavepackets defined by wave vectors and dispersion relations. One difference between phonons and photons is that the energy of a phonon is a collective mechanical eigenmode (physical vibration), whereas the energy of a photon is in the field, rather than matter. Since light is an electromagnetic vibration, it can travel outside any physical material, for example radio waves travel in space to satellites. Phonons, however, need a vibrating material to exist, and therefore only exist and travel in a material, which can be gaseous, liquid, solid or plasma.
The stimulated Raman scattering process is negligible at low intensity light levels. The process is dependent on incident light intensities, and only becomes significant when the Raman material is illuminated, or pumped, with light intensities usually attainable only with a very bright laser. Once the phonon is created or annihilated, it may be itinerant (or not) and dissipates finally as heat, due to scattering from other phonons, electrons or collective modes of the media.
Different materials exhibit varying propensities for Raman scattering of photons. This is characterized by a spontaneous Raman scattering cross-section, which is a probability that a certain length of material will scatter incoming light. Within a material there are often several types of phonons or vibration modes, each with a specific energy. Often one (or a few) phonons have Raman scattering cross-sections that dominate the rest, so the majority of the incident light energy excites one (or a few) phonon eigenmodes, so the scattered light is at one (or a few) new wavelength(s). If enough photons are scattered, they can start enhancing the formation of even more scattered photons, called Stokes (red shift) or anti-Stokes (blue shift) photons, which is called stimulated Raman scattering. This stimulated process can convert nearly all of the pump or incident light into scattered or Stokes light. If the Stokes/Anti-Stokes light is contained between one or two mirrors, the light intensity can build up further, enhancing the stimulated conversion, thus forming a Raman laser. FIG. 1 shows an example of a prior art collinear pumped Raman laser 100. A Raman material 102 has a Raman resonator placed around it consisting of aligned mirrors 104 and 106. A pump beam 108 enters through mirror 104, which is highly transmissive at the pump wavelength and highly reflective at the Raman wavelength. A stimulated Raman beam 110 is produced and is partially transmitted through mirror 106, a Raman laser output coupler (that is, a mirror partially transparent at the Raman wavelength).
The prior art in noncollinear pumped Raman lasers has consisted entirely of using gas and liquid Raman media. Gases that exhibit Raman scattering include hydrogen, a simple two-atom molecule. Liquids that scatter include nitrogen, which also has a diatomic molecule. In gases and liquids, the molecules that vibrate are not held at specific positions in space, and are continually moving, colliding and rotating, ensuring that any pump light incident upon the fluid encounters molecules at all possible orientations. In a solid, different pump light directions will encounter different molecular orientations, which can cause the scattering efficiency to change with pump light direction.
Polarization of photons and phonons occurs differently. For a photon, polarization is the direction of the vibration of the electromagnetic field as the photon travels, which can vary but it is always at right angles to the photon movement. For a phonon, there is no restriction on the angle between the direction of movement and the direction of vibration. If the vibration is along the overall phonon direction, the phonon is called a longitudinal optical or LO phonon. If the vibration is at right angles to the wave vector, like a photon, the phonon is called a transverse optical or TO phonon. Phonons can also be partially TO and LO if the polarization is at some acute angle to the wave vector, for example 45 degrees. A TO phonon can in some cases be considered similar in behavior to a photon. Optical phonons are described as xe2x80x9cRaman activexe2x80x9d, as they contribute to Raman scattering, but TO phonons in particular can in some materials also be xe2x80x9cinfrared activexe2x80x9d. The latter type of TO phonon can also be called a polariton, if it is exceptionally strongly coupled to the radiation field; the polariton exhibits similar behavior to photons. This invention is only concerned with Raman active and infrared inactive optical phonons, which are constant in frequency as the angle between the pump and Raman laser beams change in the Raman material.
An acoustic phonon is a delocalized vibration throughout the material, similar to acoustic waves, and an acoustic phonon does not contribute to Raman scattering. Acoustic phonons behave similarly to optical phonons, except that acoustic phonons tend to have much smaller energies. The acoustic phonon conversion process is often termed Brillouin Scattering. Noncollinear Brillouin scattering also occurs, much as noncollinear Raman scattering does, but it tends to involve phase-matched scattering.
A Raman Laser review article by A. Z. Grasyuk published in the Sov. J. Quant. Electron. Volume 4, Number 3, September 1974, on p. 272, discusses noncollinear pumping of solid state material as a possibility, without describing the type of material to be excited and without providing any enabling details of how such a device would be constructed.
In xe2x80x9cControlled Stimulated Raman Amplification and Oscillation in Hydrogen Gasxe2x80x9d, by Bloembergen, et al., published in the Journal of Quantum Electronics, Vol. QE-3, No. 5, May 1967, the author refers to xe2x80x9ctransversexe2x80x9d or noncollinear pumping in hydrogen, but does not state that transverse pumping can be done in solid state materials.
To date, the demonstrated advantages of noncollinearly pumped Raman lasers (discussed below) have not been combined with the advantage of utilizing solid state Raman materials. Solid state materials can be much smaller in size than the equivalent gas or liquid system, and they do not require a transparent container or cell. Thus, there is a need in the art for a solid state Raman laser. There is a further need in the art for a noncollinearly pumped solid-state Raman laser. The present invention meets these and other needs in the art.
It is an aspect of the present invention to project a pump beam into a solid-state material to produce a noncollinear Raman laser beam from the solid-state material.
It is another aspect of the invention to pass the pump laser beam through the solid-state Raman material a plurality of times, which can compress the pump laser pulse length to a shorter Raman laser pulse length.
Another aspect of the invention is to integrate out wavefront irregularities of the pump beam while scattering into the Raman beam, improving propagation performance of the output Raman laser beam.
A further aspect of the invention is to project pump beams from a plurality of pump lasers into the solid state Raman material, forming a single combined Raman laser output.
The above and other aspects of the invention are accomplished in a device that comprises a pump laser transversely or noncollinearly pumping a solid state Raman gain material, with an intensity sufficient to produce a stimulated Raman laser output beam. The pump beam and the Raman beam are noncollinear when the Raman beam travels completely across the pump beam within the Raman gain material, or where the Raman beam completely separates physically, at some location in the pumped Raman gain material, from a virtual beam, which is collinear to the pump beam axis, and which has the same entrance spot size and beam divergence as the Raman beam.
The device further comprises a plurality of mirrors, or prisms, to fold the pump beam to cause the pump beam to pass through the Raman material a plurality of times. This increases the energy transferred from the pump beam to the Raman beam by lowering the Raman laser threshold and providing additional spatial and temporal averaging of the pump laser beam.